Well, if you're used to manipulating files through the main window display (rather than windows explorer):
First - unzip the file to your desktop.
Second - if it is a full mod (like P&N) then click on the folder and see if it has a folder inside with the mod name you can pull out the folder to the desktop. If you click the folder and you see other folders like "Data", "Design Names", "Pictures" then you are already in good shape.
Third - double click down this line: "My Computer", "C Drive", "Program Files", "Shrapnel Games", "Malfador Machinations", "SE IV Gold" - this may be different if you chose to install the game elsewhere, but odds are this is where it will be.
Forth - Drag the folder into the SE IV Gold folder.
Fifth - make sure you install Modchooser to that folder as well, and drag a shortcut to modchooser out to your desktop. Now you can use Modchooser to select which mod you want to play without having to mess with the Path file.
---- Now if you want to add a race to the existing game ----
First - as above.
Second - as above, except the folder may just have a race name in it (such as Shadri) or it may have folders called "Design Name", "Empire" and "Picture"
Third - as above.
Forth - If there are seperate folders, drag the contents of the folder into the corresponding folders in the root of SE IV Gold - make sure to drag the contents and not the folder, otherwise you will have a folder in the folder that the game will not find. If all you see is a race foler with pics and AI in it, then you will need to continue to open "Pictures" then drag the new race folder into the "Races" folder.
Fifth - Play the standard game, those races should now be available. You may need to use the neostandard VehicleText file to see extra ships in a set.
Hope this isn't as confusing as it looks